Prosthetic vascular implants, such as heart-valves, stents, grafts and stent-grafts used for human implantation are subjected to the continuous fluctuating stress of blood pressure. It is therefore necessary to test such implants to prove their durability over a lifetime of exposure to pulsatile blood pressure.
A number of prior art documents disclose destructive methods of testing non-resilient vessels (such as glass bottles) by inserting a resilient insert such as a bladder into the vessel and subjecting the bladder to extremely high expansive pressure to see if the vessel breaks (see for example U.S. Pat. No. 3,895,514; GB 2,177,220; GB 1,531,557; and GB 2,149,126). None of these methods would be suitable for the pulsatile testing of a vascular implant.
Commercial machines for pulsatile testing of vascular implants are available from suppliers such as Enduratec Inc and Dynatek Dalta. These provide one or more resilient tubes into which the implant is placed. The tubes are filled with liquid, typically isotonic saline, and the pressure within the tube is varied by means of a pump. Different types of pump are used, some workers employ positive displacement mechanical pumps while others prefer electrically driven linear motors which drive pistons directly.
The fatigue process relies upon a first raised liquid pressure inside the tube expanding the tube and a second, lowered, liquid pressure allowing the tube to contract. As the tube expands, the radial resilience of the implant causes it to expand with the tube. As the tube contracts, it squeezes the implant back to its original size.
A similar method is disclosed in DE 199 03 476 (Inst. Implantatechnologie) which relates to a method of testing blood vessel implants by placing them inside an elastic sheath and subjecting them to external pressure.
There are a number of common failures or difficulties associated with locating the implant within a tube.
The most serious commercially is the consequence of a tube rupturing during the test. In most circumstances, a catheter or similar tube is employed to insert the implant into the tube. The implant is first crushed before passing through the catheter and this crushing process can severely affect the life expectancy of the implant. Thus, if the tube fails during a test, the tube itself can be replaced but it is not feasible to re-deliver the implant through a catheter. This is because it would involve crushing the implant into a catheter a second time and its life expectancy will consequently be reduced. The cost of replacing the implant is rarely important. However, endurance tests typically last between 3 and 6 months and such a failure can easily delay testing, and therefore the time to launch a product, by several months.
As a consequence of the above failure mode, designers of test machines will usually employ a particularly tough tube with thick walls. The compliance of such a tube (i.e. the percentage increase in diameter per unit pressure) is relatively low, and in order to achieve changes in diameter which are physiologically representative, the pulsatile pressures used to inflate the tubes are usually significantly higher than physiological pressures. For instance, in the abdominal aorta, blood pressure in the average healthy subject is 120 mm Hg/80 mm Hg, i.e. the blood pressure varies by 40 mm Hg for every pulse. Compliance of a healthy aorta can be of the order of 5% per 100 mm Hg so that a change in diameter of 2% can be expected at every heart beat. In order to simulate such a change in diameter, some workers employ a pulse pressure between 80 mm Hg and 100 mm Hg.
If the implant presents a significant surface area across the lumen of the vessel, such as a tapered stent or stent graft, then the force per unit area along the axis of the implant is increased in proportion with the inflation pressure of the tube. This elevated pressure induces failure modes such as limb separation or migration which would not occur at physiological pressures.
A shortcoming of existing designs unrelated to the failure described above lies in the limitation of the form of the tube. Stent grafts frequently are designed for use in bifurcated vessels and require bifurcated test tubes for their testing. Stent-grafts are also intended for use in aneurysms. Accordingly, where the vessel is normal, parts of the implant will be in contact with the wall of the vessel, whereas where the vessel is aneurysmal, the implant will be passing through a void. Moreover, diseased vessels are frequently highly tortuous. As a result of these factors, tubes must be available that bifurcate, that have different compliance in different places, that can be aneurysmal and which are highly tortuous. The production of such complex tubes is, even where possible, difficult, expensive and time consuming.
A further issue in endurance testing arises from the need to complete life-time tests in a commercially appropriate period of time. Typically, vascular implants are tested for 400,000,000 cycles which represent approximately 10 years of implantation life at a heart rate of 80 beats per minute. Many companies test large implants at approximately 35 Hz, allowing testing to be completed in approximately 19 weeks.
It is desirable to increase the speed of testing by as much as possible in order to accelerate the time taken to bring a new product to market. However, the testing method described above has a frequency limit which arises from the radial resilience and the surface area of the implant. This arises from the following mechanism:
When the pressure in the tube is increased, it moves away from the walls of the implant. The radial resilience of the implant causes the wall of the implant to follow the wall of the tube. However, this resilience may not be sufficient to overcome the frictional drag of the fluid through which the implant wall must move and the implant wall is likely therefore to move more slowly than the wall of the tube. Thus, where the drag is high, the radial resilience low and the testing speed is also high, the vascular implant can lag behind the movement of the wall of the tube. In these circumstances, the strain induced in the implant reduces as the frequency increases and the change in diameter of the implant no longer matches the change in diameter of the tube.